This method and apparatus relates to electrostatic image development and more particularly to an improved xerographic method and apparatus for the development of electrostatic images by which a toner layer is presented to the latent image for development thereof.
In the reproduction process of xerography, a photoconductive surface is charged and then exposed to a light pattern of the information to be reproduced, thereby forming an electrostatic latent image on the photoconductive surface. Charged toner particles, which may be finely divided, pigmented, resinous material are presented to the latent image where they are attracted to the photoconductive surface. The toner image can be fixed and made permanent on the photoconductive surface or it can be transferred to another surface where it is fixed.
One known method of developing electrostatic latent images is by a process called transfer development. Transfer development broadly involves bringing a layer of toner to an imaged photoconductor where toner particles will be transferred from the layer to the imaged areas. In one transfer development technique, a layer of charged toner particles is applied to a donor member which is capable of retaining the particles on its surface and then the donor member is brought into close proximity to the surface of the photoconductor. In the closely spaced position, particles of toner in the toner layer on the donor member, are attracted to the photoconductor by the electrostatic charge on the photoconductor opposite to the toner charge so that development takes place. In this technique the toner particles must traverse an air gap to reach the imaged regions of the photoconducor. In the two other transfer techniques the toner-laden donor actually contacts the imaged photoreceptor and the minimum air gap is reduced to zero in the center of the nip. In one such technique, the toner-laden donor is rolled in non-slip relationship into and out of contact with the electrostatic latent image to develop the image in a single rapid step. In another such technique, the toner-laden donor is skidded across the xerographic surface. Skidding the toner by as much as the width of a thin line will double the amount of toner available for development of the line if it lies perpendicular to the skid direction. The amount of skidding can be increased to achieve greater density or greater area coverage.
It is to be noted, therefore, that the term "transfer development" is generic to development techniques where (1) the toner layer is out of contact with the imaged photoconductor and the toner particles must traverse an air gap to development, (2) the toner layer is brought into rolling contact with the imaged photoconductor to effect development, and (3) the toner layer is brought into skidding contact with the imaged photoreceptor eo effect development. Transfer development has also come to be known as "touchdown development."
In a typical transfer development system, a cylindrical or endless donor member is rotated so that its surface can be presented to the moving surface of a photoconductive drum bearing an electrostatic latent image thereon. Positioned about the periphery of the donor member are a number of processing stations including, a donor loading station, at which toner is presented to and coated on the donor member surface; an agglomerate removal station at which toner agglomerates and excess toner are removed from the toner layer retained on the surface of the donor member; a charging station at which a uniform charge is placed on the particles of toner retained on the donor surface; a clean up station at which the toner is converted into one of uniform thickness and uniform charge state at which any toner agglomerates not removed by the agglomerate removal station are removed, a development station at which toner particles carried by said donor member are presented to the imaged photoconductor for image development; and a cleaning station at which a neutralizing charge is placed upon the residual toner particles and at which a cleaning member removes residual toner from the peripheral surface of the photoreceptor. In this manner, a continuous development process is carried out.
Among the donor members employed in the prior art are those embodying the principles described in U.S. Pat. No. 3,203,394. Such a donor includes, an electrically conductive support member in the form of a cylinder, a thin electrically insulating layer overlying a support member, and a continuous, electrically conductive screen pattern is provided. An electrical connection to a slip ring is provided so that its potential may be varied between ground potential and a charge potential at different stages of process. A multitude of high fringe fields or microfields are created at the surface of this type of donor member. When this type of donor member is brought into contact with toner particles, it is in this manner loaded with toner.
A donor member of this type is quite expensive to manufacture, it is quite fragile in the screen regions and is subject to be electrically disabled, e.g., through shorting of the screen to the conductive substrate, unless considerable care is taken during its manufacture and use.
The art of xerographic development, and in particular transfer development, would be significantly advanced if a simpler and more reliable uniform loading of the donor member and reloading in a single pass after large solid image areas have been developed out as well as a less costly system were available.
Doctor blades have been used extensively in the past for loading donors with charged toner. The blades usually are positioned at a 9 o'clock position extending toward the 11 o'clock position of a donor member and have a pointed end directed toward the surface of the donor member in order to load toner onto the surface of the donor member. In practice, large agglomerates or other debris will be wedged into the nip formed between the doctor blade and the donor member and cause streaks in the toner layer formed on the donor member.
In order to get away from this particular problem, a number of prior art devices switched to magnetic toner touchdown development with the use of magnetic donor rolls and magnetic toner. For example, Japanese Patent Document No. 52-25642 appears to show a development device that supplies a mass of toner smoothly to a developer roll by swinging a magnetic piece in a hopper due to a rotating magnetic field of a magnet roll. U.S. Pat. No. 4,370,056 discloses a belt development system which incorporates a metering blade and a paddle wheel which advances developer material to a developer roll. The metering blade is positioned to trim toner to a certain height on a toner transport belt once a strong magnetic field has been applied to the toner. A development apparatus that forms a thin toner layer on the surface of a non-magnetic sleeve that covers a fixed magnet is shown in U.S. Pat. No. 4,583,490. A magnetic blade is inclined with respect to a line normal to the surface of the sleeve in order to trim a mixture of non-magnetic and magnetic toner to a desired height on the sleeve. In U.S. Pat. No. 4,177,757, a magnetic brush development device is disclosed that incorporates multiple magnetic brushes to deliver toner to a development zone and a doctor blade for controlling the thickness of toner pumped up to the brushes. U.S. Pat. No. 4,575,220 shows a developing device in which a pressure blade is firmly pressed against a sleeve of a magnetic donor roll. The blade is comprised of a magnetic material and is magnetically attracted to a magnet disposed inside the sleeve. These magnetic development systems are cumbersome and costly and have been found to be not entirely satisfactory.